A specific 55,000 MW phosphoprotein was found in murine embryo cells. This protein is very similar, if not identical, to an SV40 induced protein, isolated earlier from SV40 transformed cultured fibroblasts. The amount of the embryo phosphoprotein is highest in cells taken from midgestation stage mouse embryos. We are studying the biochemical nature and the possible function(s) of this embryonal protein. This (or similar) protein(s) appear in cultured cells which were transformed by various other means including RNA viruses, and also in much lower amounts in normal or spontaneously transformed cells. The presence of protein in high amounts is not a general correlate of cellular tumorigenicity.